This invention relates to recovering oil from a subterranean oil reservoir by means of a conductively heated in-situ steam drive process. More particularly, the invention relates to treating a subterranean oil reservoir which is relatively porous and contains significant proportions of both oil and water but is so impermeable as to be productive of substantially no fluid in response to injections of drive fluids such as water, steam, hot gas, or oil miscible solvents.
Such a reservoir is typified by the Diatomite/Brown Shale formations in the Belridge Field. Those formations are characterized by depths of several hundred feet, thicknesses of about a thousand feet, a porosity of about 50%, an oil saturation of about 40 percent, an oil API gravity of about 30 degrees, a water saturation of about 60 percent--but a permeability of less than about 1 millidarcy, in spite of the presence of natural fractures within the formations. Those formations have been found to yield only a small percentage of their oil content, such as 5 percent or less, in primary production processes. And, they have been substantially non-responsive to conventional types of secondary or tertiary recovery processes. The production problems are typified by publications such as SPE Paper 10773, presented in San Francisco in March, 1982, on "Reasons for Production Decline in the Diatomite Belridge Oil Field: A Rock Mechanics View", relating to a study undertaken to explain the rapid decline in oil production. SPE Paper 10966 presented in New Orleans in September, 1982, on "Fracturing Results in Diatomaceous Earth Formations South Belridge Field California" also discusses those production declines. It states that calculated production curves representative of the ranges of the conditions encountered indicate cumulative oil recoveries of only from about 1-14 percent of the original oil in place.
A conductive heat drive for producing oil from a subterranean oil shale was invented in Sweden By F. Ljungstroem. That process (which was invented in the 1940s and commercially used on a small scale in the 1950s) is described in Swedish Pat. Nos. 121,737; 123,136; 123,137; 123,138; 125,712 and 126,674, in U.S. Pat. No. 2,732,195 and in journal articles such as: "Underground Shale Oil Pyrolysis According to the Ljungstroem Method", IVA Volume 24 (1953) No. 3, pages 118-123, and "Net Energy Recoveries for the In Situ Dielectric Heating of Oil Shale", Oil Shale Symposium Proceedings 11, page 311-330 (1978). In that process, heat injection wells and fluid producing wells were completed within a permeable near-surface oil shale formation with less than a three meter separation between the boreholes. The heat injection wells were equipped with electrical or other heating elements which were surrounded by a mass of material (such as sand or cement) arranged to transmit heat into the oil shale while preventing any inflowing or out-flowing of fluid. In the oil shale for which the process was designed and tested, a continuous inflowing of ground water required a continuous pumping out of water to avoid an unnecessary wasting of energy in evaporating that water.
U.S. Pat. No. 3,113,623 describes means for heating subterranean earth formations to facilitate hydrocarbon recovery by using a flow reversal type of burner in which the fuel is inflowed through a gas permeable tubing in order to cause combustion to take place throughout an elongated interval of subterranean earth formation.
With respect to substantially completely impermeable, relatively deep and relatively thick, potentially oil-productive deposits such as tar sands or oil shale deposits, such as those in the Piceance Basin in the United States, the possibility of utilizing a conductive heating process for producing oil would surely be--according to prior teachings and beliefs--economically unfeasible. For example, in the above-identified Oil Shale Symposium the Ljungstroem process is characterized as a process which " . . . successfully recovered shale oil by embedding tubular electrical heating elements within high-grade shale deposits. This method relied on ordinary thermal diffusion for shale heating, which, of course, requires large temperature gradients. Thus, heating was very non-uniform; months were required to fully retort small room-size blocks of shale. Also, much heat energy was wasted in underheating the shale regions beyond the periphery of the retorting zone and overheating the shale closest to the heat source. The latter problem is especially important in the case of Western shales, since thermal energy in overheated zones, cannot be fully recovered by diffusion due to endothermic reactions which take place above about 600.degree. C. (page 313).
In substantially impermeable types of subterranean formations, the creating and maintaining of a permeable zone through which the heated oil or pyrolysis products can be flowed has been found to be a severe problem. In U.S. Pat. No. 3,468,376, it is stated (in Cols. 1 and 2) that "There are two mechanisms involved in the transport of heat through the oil shale. Heat is transferred through the solid mass of oil shale by conduction. The heat is also transferred by convection through the solid mass of oil shale. The transfer of heat by conduction is a relatively slow process. The average thermal conductivity and average thermal diffusivity of oil shale are about those of a firebrick. The matrix of solid oil shale has an extremely low permeability much like unglazed porcelain. As a result, the convective transfer of heat is limited to heating by fluid flows obtained in open channels which traverse the oil shale. These flow channels may be natural and artificially induced fractures . . . . On heating, a layer of pyrolyzed oil shale builds adjacent the channel. This layer is an inorganic mineral matrix which contains varying degrees of carbon. The layer is an ever-expanding barrier to heat flow from the heating fluid in the channel." The patent is directed to a process for circulating heated oil shale-pyrolyzing fluid through a flow channel while adding abrasive particles to the circulating fluid to erode the layer of pyrolyzed oil shale being formed adjacent to the channel.
U.S. Pat. No. 3,284,281 says (Col. 1, lines 3-21), "The production of oil from oil shale, by heating the shale by various means such as . . . an electrical resistance heater . . . has been attempted with little success . . . . Fracturing of the shale oil prior to the application of heat thereto by in situ combustion or other means has been practiced with little success because the shale swells upon heating with consequent partial or complete closure of the fracture." The patent describes a process of sequentially heating (and thus swelling) the oil shale, then injecting fluid to hydraulically fracture the swollen shale, then repeating those steps until a heat-stable fracture has been propagated into a production well. U.S. Pat. No. 3,455,391 discloses that in a subterranean earth formation in which hydraulically induced fractures tend to be vertical fractures, hot fluids can be flowed through the vertical fracture to thermally expand the rocks and close the fractures so that fluid can be injected at a pressure sufficient to form horizontal fractures.